1. Field of the Invention
The disclosed invention generally relates to correlation receivers, and is particularly directed to a differential correlation detector circuit.
2. Background Art
Correlating techniques have been recognized as powerful tools in signal processing, and have substantial utility in communications applications ranging from multiplexed communications to secure communications. Such signal processing is based on correlation of an information carrying signal with a reference signal. if the information signal is identified as S and the reference signal is identified as P, then the correlation function F over a time period T is as follows: ##EQU1##
A specific application of the correlation function is a correlation detector wherein the information signal S is a received analog waveform and the reference waveform P is a known digital signal with a period T. In such a correlation detector, often only the sign of the function F over the period T is needed, thereby providing a binary output wherein positive indicates "one" and negative indicates "zero".
In synchronized systems the waveforms S and P are synchronized whereby they begin at the same time. However, the application of correlation detection extends to systems wherein S and P are not necessarily synchronized. More specifically, if the limit of integration is fixed to the start of the period of the waveform P and the waveform S is shifted in time relative to the waveform P, the function F can then be computed for a range of time shifts between the waveforms S and P. If the waveforms S and P are correlated (that is, the pattern of the signal S in some way resembles the pattern of the signal P) then the function F will be a maximum when the patterns coincide. Since the signals S and P are synchronized when they coincide, the function F is a maximum when the signals S and P are synchronized. Clearly, the foregoing process can be utilized to achieve synchronization between the signals S and P.
Further elaborating on the foregoing, one type of correlation detector utilizes what is known as the "spread spectrum" technique. In a particular class of spread spectrum transmitters, information D is multiplied by a digital signal (pattern) V to provide the transmitted signal TR. EQU TR=D.multidot.V (Equation 2)
The pattern of binary ones and zeroes that form the digital signal V are typically defined by a predetermined code. The digital signal V has a period T, and the information D does not change during such period.
At the receiver, the information D can be recovered from a received signal S provided certain conditions are satisfied. An appropriate signal P is provided which satisfies the following: EQU P=V (Equation 3) ##EQU2##
The received signal S is synchronized with the signal P, and the information D can be recovered by the correlation of S with P, provided that the information D is unchanged at the transmitter over each period T. Thus, the information D is provided by: ##EQU3##
Common practice for accomplishing correlation detection includes the use of a four quadrant analog multiplier for multiplying the signals S and P, and an integration circuit for integrating the product of the signals over the integration period T. Such an integration circuit is often referred to as an "integrate and dump" circuit since the integration result of an integration interval must be discarded prior to integration over the next integration interval. The output of the integration circuit is sampled by an output amplifier at the end of the integration interval T. When only the sign of the integration output is desired, the output amplifier may be a comparator referenced to zero to detect sign.
Correlation circuitry for accomplishing multiplication and integration are generally complex and not readily adaptable to large scale integrated circuitry for a number of reasons.
Analog multipliers generally include operational amplifiers and logging transistors, and may include potentiometers to null the offset voltages of the logging transistors. Further, for known analog multipliers frequency compensation is difficult to achieve, and high speed operation is also difficult to achieve.
While the concept of integration over a period T is straightforward, implementation of integration circuitry is not. Typically, integration is accomplished with an operational amplifier configured as an integrator or with a gated current source. In any case, a capacitor integrates a controlled current, and the capacitor voltage at the end of the integration period is indicative of the integration result and is sampled. The capacitor voltage is then reset to a predetermined level for the next integration period.
Known integration circuits are typically single-ended integrators wherein a single capacitor integrates over any given integration period. Therefore, the capacitance values of the integration capacitors must be precisely achieved with absolute tolerances, which cannot be readily achieved with LSI techniques. Further, such single ended integration requires the use of bidirectional current sources which are difficult to implement. As a result, different techniques have been developed to accomplish positive and negative charging of an integration capacitor.
Also, known single-ended integration circuits exhibit sensitivity to temperature whereby different operating temperatures cause different integration results. Compensation for temperature sensitivity requires additional complicating circuitry.
Another disadvantage of known integration circuits, particularly those implemented with operational amplifiers, is that they cannot be readily utilized for high speed processing.
Since known correlation detection circuits generally include multipliers and integration circuits, they have the disadvantages of known multipliers and integration circuits. Thus, known correlation detection techniques are generally not readily adaptable to large scale integration, nor can they be utilized in very high speed applications. Moreover, because of complexity and unsuitability to integration, known correlation detection circuits are not readily utilized in applications which might advantageously utilize correlation detection but would require numerous correlation detection circuits in a single receiver. An example of such application would be a high speed multiplexed communications system.
Perhaps as a result of implementation difficulties, correlation techniques have not been widely utilized in communications, despite the recognition that correlation techniques are powerful signal processing tools.